Fossil fuel powered steam generators, nuclear powered steam systems, and the like, employ many thousands of pipes through which water flows at high temperature and high pressure, e.g., boiler tube headers. The water within the pipe may flow at perhaps 1 m.sup.3 /second, at 600.degree. F. or more, and at pressures of perhaps 1,100 psi. In a nuclear steam system, the outer surface of the pipe may be exposed to liquid at perhaps 2,500 psi.
Unfortunately, cracking and corrosion of the pipes, perhaps due to concentration of chemical impurities in the water at pipe crevices, can reduce the strength of the pipe walls. Unless detected sufficiently early, pipe failure and costly steam generator down-time can result. The failure mechanism appears to result from progressive intergranular attack and stress corrosion cracking, or "IGA/SCC". Unfortunately, it is difficult to reliably measure or successfully predict IGA/SCC failure on a long term basis within the hostile environment typified by the steam generators.
For example, it is known in the art to monitor strain at high temperatures using non-contact strain measuring techniques. But these devices are poorly suited for long-term measurements on power generation components, whose surfaces are often insulated and/or are subject to degradation. Further, the pipes to be monitored are frequently located in regions where visual interrogation is difficult or impossible. Also, non-contact strain measurement devices are affected by temperature, opacity, and the turbulence of any intervening atmosphere.
Contact strain gages such as electrical resistance gages are also known in the art and can sense strain at temperatures exceeding 700.degree. F. on a long-term basis, and at even higher temperatures for short-term or dynamic measurements. For example, bonded resistance gages are commonly used continuously at temperatures up to 500.degree. F. and have relatively high compliance, e.g., the ability to readily conform to the surface of the object under measurement.
So-called "Eaton" and "Kyowa" weldable resistance gages may be used at such temperatures, but have less compliance due to package stiffness. In the 600.degree. F. to 650.degree. F. range, such gages are made with a modified nickel-chrome alloy that has good drift characteristics, relative small apparent strain, and repeatable apparent strain characteristics. As used herein, "drift" refers to the stability of the strain gage output, while "apparent strain" refers to the change in output of the strain gage as a function of temperature in a regime in which hysteresis effects do not predominate. But it is difficult to adequately temperature compensate such devices using heat treatment techniques. Above 650.degree. F., the sense material undergoes a metallurgical phase transition that can "reset" the temperature compensation, causing radical zero shifts.
In the 1000.degree. F. to 1100.degree. F. range, it is difficult to retain calibration, especially with resistance strain gages. Apparent strain, drift, and hysteresis due to temperature cycling present problems. At present, it is not known how to accomplish long-term static strain measurement at such elevated temperatures.
The drift problem has been somewhat addressed in the prior art using high-temperature capacitive strain gages. But such devices are not generally suitable for dynamic measurements above 100 Hz. Although low drift characteristics enable capacitive strain gages to measure creep strain change at steady-state, installation, calibration and other documentation can be costly. Nonetheless, capacitive type gages represent the only presently available contact devices useable for field measurement of creep strain at temperatures above 1000.degree. F.
All of the above-described gages suffer the common problem of requiring electrical connections, which frequently are difficult to implement in a power plant generator environment. Further, electrical connections can act as antennae that render the gages susceptible to electromagnetic noise that can adversely affect the measurements. By contrast, fiber optic strain gages do not require such connections, and are useable at temperatures exceeding 700.degree. F., and in fact can survive up to about 2,000.degree. F.
A microbend fiber optic type strain gage is described in U.S. Pat. No. 5,020,379 to Berthold, in which a strain sensing optical fiber is sandwiched between a pair of tooth-edged end plates. Strain causes the end plates to move toward or away from each other. This causes the fiber to become deformed, which amplitude modulates a light signal transmitted through the fiber. This modulation is detected to provide strain information having excellent resolution. A second, equal length reference optical fiber may be sandwiched between a similar pair of tooth-edged end plates that are locked to each other but not attached to the structure. As such, the reference fiber compensates for source brightness variations and changes in fiber transmission over time, thus providing temperature compensation.
The Berthold device is similar to a conventional strain gage in that it modulates the "resistance" to light passing through the sensing fiber. The device is also similar to a conventional capacitance gage in that it relies upon relative movement of two plates for its measurement. However, unlike the capacitance gage, the microbend fiber stiffness makes the gage less compliant than capacitance gages, but still more compliant than weldable resistance gages. The microbend fiber optic strain gage can provide stable and extended life data at temperatures up to 1100.degree. F.
One drawback with intensity-modulated sensors such as the Berthold device is that there is a light intensity loss due to couplers, fiber microbending, and other attenuation contributors along the length of the optical fiber. Because these losses degrade accuracy and reliability of the stress and temperature related signals to be measured, such devices are not well suited for long-term monitoring applications.
It is also known in the art to use phase-modulated optical fiber devices in an interferometric configuration. Intrinsic and extrinsic Fabry-Perot fiber optic interferometric sensors are useful for measuring localized strain (e.g., gage length of perhaps 1 mm to 10 mm) on a relative but not absolute basis. For example, a white light optical source is coupled into one of two branches of a 2:1 multimode fiber coupler/splitter. The main portion of the fiber is mounted with a tube and is spaced-apart by an air gap from a high resolution Fabry-Perot sensor.
FIG. 1 is a simplified cross-sectional depiction of such an extrinsic Fabry-Perot strain sensor 2 mounted with adhesive 4 to structure 6 for detection of strain within the structure by means of a detector 8. The sensor has a "Y"-shaped transmit/receive fiber optic structure 10 includes a first fiber optic branch 12 that receives light 14 from a preferably white source 16. (For ease of illustration, FIG. 1 does not depict a protective buffer coating over branches 12 and 32 of structure 10.) The light travels to surface 18 of the fiber optic, traverses an air gap 20 and reflects, at least partially, from surface 22 of a target fiber 24. The sensor structure typically includes a quartz outer tube 26 that is fused at joints 28 to the underlying transmit/receive and target fiber optic members. (Epoxy 30 or the like joins tube 26 to reflector fiber 24.) The lateral distance between these fused joints 28 defines a dimension AL that will decrease slightly as the strain sensor is subjected to tension, and that will increase slightly as sensor 2 is subjected to compression.
Depending upon the lateral separation of the air gap 20, some light frequencies within light energy 14 will reflect more strongly than others from target fiber surface 22. These wavelengths traverse branch 32 of "Y"-shaped structure 10 and are presented to detector 8. Stated differently, if strain causes changes to the air gap dimension, the strain may be characterized by a difference in the frequency or frequencies that are most strongly reflected. These frequency differences may be detected with a spectrum analyzer detector 8, to provide a measure of the changing value of .DELTA.L, and thus to measure the strain to which sensor 2 was subjected.
One difficulty with such devices is that constructing the sensor requires modifying the optical fibers. Further, attaching, in the field or otherwise, the glass fiber optical sensor to the typically metallic structure that is to be temperature monitored is not trivial, especially at higher temperatures. Adhesives, e.g., adhesive 4 in FIG. 1, available for attaching the strain sensor to such structures cannot survive temperatures exceeding about 600.degree. F.
One prior art technique for attaching a fiber optic strain sensor to a metal structure is to plasma spray molten metal to encapsulate the strain sensor and thus to attach it to the surface of the metal structure to be monitored. Unfortunately, the force exerted by the plasma spray upon the fiber-optic is severe, and can damage the sensor even before the plasma attachment process has ended. Further, plasma spraying does not lend itself to field-installation of the fiber optic sensors.
Another prior art technique for attaching fiber optic strain sensors to a metal structure uses high temperature ceramic adhesive. However, the thermal expansion coefficient for such adhesive is so dissimilar from the coefficient for the metal structure, that failed attachment joints can result.
Further, whether plasma spraying or ceramic adhesives are used, it is simply not convenient to attach a fiber optic strain sensor to a structure in the field.
In short, while phase-modulated fiber optic strain sensors can provide long term measurements at elevated temperatures, there is a need for a method and mechanism for producing such sensors such that they can be field-mounted to a metal structure to be monitored.
The present invention provides a method and mechanism for providing such predictions.